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proc.5: Document that 'iowait' field of /proc/stat is unreliable 

Text taken from Chao Fan's kernel commit 9c240d757658a3ae996.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2017-05-22 14:01:33 +02:00
parent 3d5ea0476d
commit e0a73a311f
1 changed files with 16 additions and 0 deletions
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@ -3730,6 +3730,22 @@ pseudo-file.
.TP
.IR iowait " (since Linux 2.5.41)"
(5) Time waiting for I/O to complete.
This value is not reliable, for the following reasons:
.\" See kernel commit 9c240d757658a3ae9968dd309e674c61f07c7f48
.RS
.IP 1. 3
The CPU will not wait for I/O to complete;
iowait is the time that a task is waiting for I/O to complete.
When a CPU goes into idle state for outstanding task I/O,
another task will be scheduled on this CPU.
.IP 2.
On a multi-core CPU,
the task waiting for I/O to complete is not running on any CPU,
so the iowait of each CPU is difficult to calculate.
.IP 3.
The value in this field may
.I decrease
in certain conditions.
.TP
.IR irq " (since Linux 2.6.0-test4)"
(6) Time servicing interrupts.